In the printing industry, it is common to provide a sample of an image to the customer for approval prior to printing a large number of copies of the image using a high volume output device such as a printing press. The sample image is known as a “proof”. The proof is used to ensure that the consumer is satisfied with the contents, composition and color gamut and tone characteristics of the image.
It is not, however, cost effective to print the proof using high volume output devices of the type used to print large quantities of the image. This is because it is expensive to set up high volume output devices to print an image. Accordingly, it has become the practice in the printing industry to use digital color printers to print proofs. Digital color printers render color prints of images that have been encoded in the form of digital data. This data includes code values indicating the colors to be printed in an image. When the color printer generates the printed output of an image, it is intended that the image recorded on the printed output will contain the exact colors called for by the code values in the digitally encoded data.
In practice, it has been found that the colors in the images printed by digital color printers do not always match the colors printed by high volume output devices. One reason for this is that variations in ink, paper and printing conditions can cause a digital color printer to generate images with colors that do not match the colors produced by a high volume output device using the same values. Therefore, a proof printed by a digital color printer may not have colors that match the colors that will be printed by the high volume output device.
Accordingly, digital color printers have been developed that can be color adjusted so that they can mimic the performance of high volume output devices. Such adjustable color printers are known in the industry as “proofers”. Two types of adjustments are commonly applied to cause proofers to produce visually accurate proofs of an image: color calibration adjustments and color management adjustments.
Color calibration adjustments are used to modify the operation of the proofer so that the proofer prints the colors called for in the code values of the images to be printed by the proofer. These adjustments are necessary to compensate for the variations in ink, paper and printing conditions that can cause the colors printed by the proofer to vary from the colors called for in the code values. To determine what color calibration adjustments must be made, it is necessary to determine how the proofer translates code values into colors on a printed image. This is done by asking the proofer to print a calibration test image. The calibration test image consists of a number of color patches. Each color patch contains the color printed by the proofer in response to a particular code value. The stand-alone calibration device measures the colors in the test image. The color of each color patch is compared to code values associated with that patch and the comparisons are used to determine what adjustments must be made to the proofer to cause the proofer to print desired colors in response to particular color code values.
Color management adjustments are used to modify the operation of the proofer so that an image printed by the proofer will have an appearance that matches the appearance of the same image as printed by a high volume output device. The first step in color management is to determine how the high volume output device converts color code values into printed colors. This is known as characterization. To characterize a high volume output device it is necessary to obtain a characterization test image. The characterization test image can be printed by the high volume output device. However, if it is known that the high volume output device converts code values into printed colors in accordance with an industry standard proofing system such as MatchPrint ™ or Cromalin ™, then a test image printed in accordance with that standard can be used for characterization purposes.
In either case, the characterization test image is submitted to the stand-alone color management device. The color patches on the characterization test image are compared to the color code values associated with the patches. This comparison is used to determine the adjustments that must be made to cause the proofer to print images having the same color gamut and tone characteristics as the images printed by the high volume output device. The proofer is then adjusted accordingly.
In this manner, the proofer is adjusted so that the proofer is properly calibrated to render images having the colors called for in the code values in the image to be proofed and is also adjusted to modify the code values in the image to be proofed in accordance with the profile for the output device. Thus, the proofer renders images having the colors that will appear the same as the colors in the images printed by output device.
It will be recognized that both calibration adjustments and color management adjustments are based upon objective measurements of the color gamut and tone characteristics of the test images printed by the proofer and by the high volume output device.
Various devices are used to measure the color content of an image. The most common devices are the densitometer and the color scanner. These devices typically analyze the color content of the light reflected by an image by dividing light into a set of primary colors, such as red, green and blue. These devices divide light into primary colors by passing the light through a set of colored filters. By measuring the intensity of the light in each primary color, it is possible to objectively measure the color content of an image.
A special form of densitometer, the colorimeter, can also be used to objectively measure the color gamut and tone characteristics of an image. Colorimeters are designed to objectively measure the color of a sample in a way that approximates human visual response. This is accomplished by the use of filters that are chosen to mimic human visual response.
A more accurate device for measuring color for calibration and color management purposes is the spectrophotometer. The spectrophotometer measures the reflectance or transmittance of an object at a number of wavelengths throughout the visible spectrum. More specifically, a spectrophotometer exposes a test image to a known light source and then analyzes the light that is reflected by the test image to determine the spectral intensity of the sample. A typical spectrophotometer is capable of measuring a group of pixels in an image and includes an apparatus that measures the light that is reflected by a portion of an image at a number of wavelengths throughout the visible spectrum to obtain data that reflects the true spectral content of the reflected light. Because the spectrophotometer measures color with greater accuracy than do the other measurement devices discussed above, the spectrophotometer is preferred.
Thus, densitometers, colorimeters, color scanners, and spectrophotometers can be used for color measurement. However, these are typically stand-alone devices and the use of such devices during proofing is very costly. Part of this cost is created by the inherent redundancy of many of the systems used in these devices. For example, a stand-alone spectrophotometer, has an “X-Y” table to move the test image relative to the spectrophotometer. A digital color printer or proofer also contains an “X-Y” displacement mechanism for moving the paper and printing element or printhead. Similarly, both the spectrophotometer and the proofer contain separate electrical control systems, motors and other components. Thus, the total cost of the proofing system including a separate stand-alone color measurement device and a proofer is high and can be in excess of more than U.S. $10,000.00.
Installation and maintenance costs are also high because two separate devices, typically manufactured by different vendors, must be separately purchased, installed, and maintained. Finally, there is a significant labor cost associated with making calibration and color management adjustments to the proofer using a stand-alone color measurement device.
Accordingly, there are substantial cost and efficiency penalties associated with stand-alone proofing combinations and what is needed is an integrated proofing apparatus.
Special printers having integrated color scanners or densitometers for color calibration purposes exist. Examples of color calibration and correction systems of this type can be found in commonly assigned U.S. Pat. Nos. 5,053,866, and 5,491,586. These patents show specially designed printing systems for generating a color image and adjusting the color content of subsequent images based upon the colors printed in the color image. However, these specially designed systems also use redundant structures for printing and color measurement and do not teach or suggest color management capabilities.
It will also be recognized that many high quality color digital printers exist. However, these printers are not designed with integral proofing capabilities. Thus, what is also needed is a proofing head having calibration and color management capabilities and that can be readily integrated into an existing printer.
Accordingly, it is an object of the present invention to provide a proofer that is low in cost and is easily maintained.
It is also an object to provide a proofer that substantially automates the proofing process.
It is also an object of the present invention to provide a proofing head that can be readily incorporated into a printer of conventional design to permit the printer to act as a proofer.